Isotope exchange studies of electrochemical systems with solid oxide electrolytes

Abstract

It has been shown that electronic conductivity of oxides is one of the main factors affecting the rate of interphase exchange of oxygen: these compounds, except ZrO2-based electrolytes with neglibible electronic conductivity, exchange with gas fairly actively (1016–1018 molecule/(cm2s) at 1000 K). It is possible to increase the exchange rate for ZrO2-based electrolyte by a factor of 102–103 by modifying its surface with electrode materials - metals (Pt, Ag) and oxides (PrO2−y, La0.7Sr0.3CoO3−y). It has been shown that electrochemical exchange current I0 and the rate of isotopic exchange of oxygen R are equal (in the same units) in electrode system O2, Pt/ZrO2-based electrolyte. It is due to the same reaction path connected with the Pt surface in both cases. In the electrode systems with electrolytes based on Bi2O3 and CeO2, the rate R > I0 because both the exchange on the Pt surface and the exchange on the electrolyte-gas interphase make contributions to the interphase exchange. A theoretical model have been proposed to describe the isotope exchange kinetics between gas and oxides. It makes it possible to determine not only the rate of interphase exchange and diffusivity of oxygen in oxides but also to analyze the contribution to the kinetics from various type of surface exchange taking diffusion into account. This method has been used for description of the oxygen exchange in a number of oxides (YBa2Cu3O7−y, La0.7Sr0.3CoO3−y, high-temperature protonic conductors CaTi1−xFexO3−y, YLa1−xCaxO3−y).